This document is the central repository for all information pertaining to
exception handling in LLVM. It describes the format that LLVM exception
handling information takes, which is useful for those interested in creating
front-ends or dealing directly with the information. Further, this document
provides specific examples of what exception handling information is used for
in C/C++.

Exception handling for most programming languages is designed to recover from
conditions that rarely occur during general use of an application. To that
end, exception handling should not interfere with the main flow of an
application's algorithm by performing checkpointing tasks, such as saving the
current pc or register state.

The Itanium ABI Exception Handling Specification defines a methodology for
providing outlying data in the form of exception tables without inlining
speculative exception handling code in the flow of an application's main
algorithm. Thus, the specification is said to add "zero-cost" to the normal
execution of an application.

For each function which does exception processing, be it try/catch blocks
or cleanups, that function registers itself on a global frame list. When
exceptions are being unwound, the runtime uses this list to identify which
functions need processing.

Landing pad selection is encoded in the call site entry of the function
context. The runtime returns to the function via
llvm.eh.sjlj.longjmp, where
a switch table transfers control to the appropriate landing pad based on
the index stored in the function context.

In contrast to DWARF exception handling, which encodes exception regions
and frame information in out-of-line tables, SJLJ exception handling
builds and removes the unwind frame context at runtime. This results in
faster exception handling at the expense of slower execution when no
exceptions are thrown. As exceptions are, by their nature, intended for
uncommon code paths, DWARF exception handling is generally preferred to
SJLJ.

When an exception is thrown in LLVM code, the runtime does its best to find a
handler suited to processing the circumstance.

The runtime first attempts to find an exception frame corresponding to
the function where the exception was thrown. If the programming language
(e.g. C++) supports exception handling, the exception frame contains a
reference to an exception table describing how to process the exception. If
the language (e.g. C) does not support exception handling, or if the
exception needs to be forwarded to a prior activation, the exception frame
contains information about how to unwind the current activation and restore
the state of the prior activation. This process is repeated until the
exception is handled. If the exception is not handled and no activations
remain, then the application is terminated with an appropriate error
message.

Because different programming languages have different behaviors when
handling exceptions, the exception handling ABI provides a mechanism for
supplying personalities. An exception handling personality is defined
by way of a personality function (e.g. __gxx_personality_v0
in C++), which receives the context of the exception, an exception
structure containing the exception object type and value, and a reference
to the exception table for the current function. The personality function
for the current compile unit is specified in a common exception
frame.

The organization of an exception table is language dependent. For C++, an
exception table is organized as a series of code ranges defining what to do
if an exception occurs in that range. Typically, the information associated
with a range defines which types of exception objects (using C++ type
info) that are handled in that range, and an associated action that
should take place. Actions typically pass control to a landing
pad.

A landing pad corresponds to the code found in the catch portion of
a try/catch sequence. When execution resumes at a landing
pad, it receives the exception structure and a selector corresponding to
the type of exception thrown. The selector is then used to determine
which catch should actually process the exception.

At the time of this writing, only C++ exception handling support is available
in LLVM. So the remainder of this document will be somewhat C++-centric.

From the C++ developers perspective, exceptions are defined in terms of the
throw and try/catch statements. In this section
we will describe the implementation of LLVM exception handling in terms of
C++ examples.

Languages that support exception handling typically provide a throw
operation to initiate the exception process. Internally, a throw operation
breaks down into two steps. First, a request is made to allocate exception
space for an exception structure. This structure needs to survive beyond the
current activation. This structure will contain the type and value of the
object being thrown. Second, a call is made to the runtime to raise the
exception, passing the exception structure as an argument.

In C++, the allocation of the exception structure is done by
the __cxa_allocate_exception runtime function. The exception
raising is handled by __cxa_throw. The type of the exception is
represented using a C++ RTTI structure.

A call within the scope of a try statement can potentially raise an
exception. In those circumstances, the LLVM C++ front-end replaces the call
with an invoke instruction. Unlike a call, the invoke has
two potential continuation points: where to continue when the call succeeds
as per normal; and where to continue if the call raises an exception, either
by a throw or the unwinding of a throw.

The term used to define a the place where an invoke continues after
an exception is called a landing pad. LLVM landing pads are
conceptually alternative function entry points where an exception structure
reference and a type info index are passed in as arguments. The landing pad
saves the exception structure reference and then proceeds to select the catch
block that corresponds to the type info of the exception object.

Two LLVM intrinsic functions are used to convey information about the landing
pad to the back end.

llvm.eh.exception takes no
arguments and returns a pointer to the exception structure. This only
returns a sensible value if called after an invoke has branched
to a landing pad. Due to code generation limitations, it must currently
be called in the landing pad itself.

llvm.eh.selector takes a minimum
of three arguments. The first argument is the reference to the exception
structure. The second argument is a reference to the personality function
to be used for this try/catch sequence. Each of the
remaining arguments is either a reference to the type info for
a catch statement, a filter
expression, or the number zero (0) representing
a cleanup. The exception is tested against the
arguments sequentially from first to last. The result of
the llvm.eh.selector is a
positive number if the exception matched a type info, a negative number if
it matched a filter, and zero if it matched a cleanup. If nothing is
matched, the behaviour of the program
is undefined. This only returns a sensible
value if called after an invoke has branched to a landing pad.
Due to codegen limitations, it must currently be called in the landing pad
itself. If a type info matched, then the selector value is the index of
the type info in the exception table, which can be obtained using the
llvm.eh.typeid.for
intrinsic.

Once the landing pad has the type info selector, the code branches to the
code for the first catch. The catch then checks the value of the type info
selector against the index of type info for that catch. Since the type info
index is not known until all the type info have been gathered in the backend,
the catch code will call the
llvm.eh.typeid.for intrinsic
to determine the index for a given type info. If the catch fails to match
the selector then control is passed on to the next catch. Note: Since the
landing pad will not be used if there is no match in the list of type info on
the call to llvm.eh.selector, then
neither the last catch nor catch all need to perform the check
against the selector.

Finally, the entry and exit of catch code is bracketed with calls
to __cxa_begin_catch and __cxa_end_catch.

__cxa_begin_catch takes a exception structure reference as an
argument and returns the value of the exception object.

__cxa_end_catch takes no arguments. This function:

Locates the most recently caught exception and decrements its handler
count,

Removes the exception from the "caught" stack if the handler count
goes to zero, and

Destroys the exception if the handler count goes to zero, and the
exception was not re-thrown by throw.

Note: a rethrow from within the catch may replace this call with
a __cxa_rethrow.

To handle destructors and cleanups in try code, control may not run
directly from a landing pad to the first catch. Control may actually flow
from the landing pad to clean up code and then to the first catch. Since the
required clean up for each invoke in a try may be different
(e.g. intervening constructor), there may be several landing pads for a given
try. If cleanups need to be run, an i32 0 should be passed as the
last llvm.eh.selector argument.
However, when using DWARF exception handling with C++, a i8* nullmust be passed instead.

C++ allows the specification of which exception types can be thrown from a
function. To represent this a top level landing pad may exist to filter out
invalid types. To express this in LLVM code the landing pad will
call llvm.eh.selector. The
arguments are a reference to the exception structure, a reference to the
personality function, the length of the filter expression (the number of type
infos plus one), followed by the type infos themselves.
llvm.eh.selector will return a
negative value if the exception does not match any of the type infos. If no
match is found then a call to __cxa_call_unexpected should be made,
otherwise _Unwind_Resume. Each of these functions requires a
reference to the exception structure. Note that the most general form of an
llvm.eh.selector call can contain
any number of type infos, filter expressions and cleanups (though having more
than one cleanup is pointless). The LLVM C++ front-end can generate such
llvm.eh.selector calls due to
inlining creating nested exception handling scopes.

The semantics of the invoke instruction require that any exception that
unwinds through an invoke call should result in a branch to the invoke's
unwind label. However such a branch will only happen if the
llvm.eh.selector matches. Thus in
order to ensure correct operation, the front-end must only generate
llvm.eh.selector calls that are
guaranteed to always match whatever exception unwinds through the invoke.
For most languages it is enough to pass zero, indicating the presence of
a cleanup, as the
last llvm.eh.selector argument.
However for C++ this is not sufficient, because the C++ personality function
will terminate the program if it detects that unwinding the exception only
results in matches with cleanups. For C++ a null i8* should be
passed as the last llvm.eh.selector
argument instead. This is interpreted as a catch-all by the C++ personality
function, and will always match.

This intrinsic is used to compare the exception with the given type infos,
filters and cleanups.

llvm.eh.selector takes a minimum of
three arguments. The first argument is the reference to the exception
structure. The second argument is a reference to the personality function to
be used for this try catch sequence. Each of the remaining arguments is
either a reference to the type info for a catch statement,
a filter expression, or the number zero
representing a cleanup. The exception is tested
against the arguments sequentially from first to last. The result of
the llvm.eh.selector is a positive
number if the exception matched a type info, a negative number if it matched
a filter, and zero if it matched a cleanup. If nothing is matched, the
behaviour of the program is undefined. If a type
info matched then the selector value is the index of the type info in the
exception table, which can be obtained using the
llvm.eh.typeid.for intrinsic.

This intrinsic returns the type info index in the exception table of the
current function. This value can be used to compare against the result
of llvm.eh.selector. The single
argument is a reference to a type info.

The SJLJ exception handling uses this intrinsic to force register saving for
the current function and to store the address of the following instruction
for use as a destination address by llvm.eh.sjlj.longjmp. The buffer format and the overall
functioning of this intrinsic is compatible with the GCC
__builtin_setjmp implementation, allowing code built with the
two compilers to interoperate.

The single parameter is a pointer to a five word buffer in which the calling
context is saved. The front end places the frame pointer in the first word,
and the target implementation of this intrinsic should place the destination
address for a
llvm.eh.sjlj.longjmp in the
second word. The following three words are available for use in a
target-specific manner.

Used for SJLJ based exception handling, the llvm.eh.sjlj.lsda intrinsic returns the address of the Language
Specific Data Area (LSDA) for the current function. The SJLJ front-end code
stores this address in the exception handling function context for use by the
runtime.

For SJLJ based exception handling, the llvm.eh.sjlj.callsite intrinsic identifies the callsite value
associated with the following invoke instruction. This is used to ensure
that landing pad entries in the LSDA are generated in the matching order.

An exception handling frame eh_frame is very similar to the unwind
frame used by dwarf debug info. The frame contains all the information
necessary to tear down the current frame and restore the state of the prior
frame. There is an exception handling frame for each function in a compile
unit, plus a common exception handling frame that defines information common
to all functions in the unit.

An exception table contains information about what actions to take when an
exception is thrown in a particular part of a function's code. There is one
exception table per function except leaf routines and functions that have
only calls to non-throwing functions will not need an exception table.